Growth factors have tremendous therapeutic potential for neurodegenerative disorders. However, growth factors have yet to be successfully developed into clinical treatments due to the fact that large proteins, like growth factors, do not cross the blood-brain barrier. The transplantation of cells genetically engineered to produce growth factors offers a partial solution to the problem of growth factor delivery, because grafts of growth factor-producing cells can bypass the blood-brain barrier and deliver the therapeutic factors directly to the target site. Unfortunately, these transplants are subject to host immune rejection and require immunosuppression. Also, grafts of some genetically engineered cell lines can form lethal tumors.
Implanting cells that have been macroencapsulated in semi-permeable polymer membranes provides a better solution to these problems. Mammalian cells that have been genetically engineered to produce growth factors can be encapsulated in semipermeable polymer membranes. The semipermeable membranes protect the encapsulated cells from acute host immune rejection, but allow the delivery of the therapeutic agents into the host tissue. These small bioartificial devices (cells macroencapsulated in semipermeable membranes) can be implanted directly into the target site for site-specific, continuous, long-term, low-level delivery of the desired factors. Encapsulating cells in semi-permeable membranes also reduces the risk of tumor development. Furthermore, polymer-encapsulated cell transplants have lower incidences of infection, because the transplants require only a single penetration into the target site for continuous growth factor delivery.
Regarding the delivery of desired growth factors, pre-clinical studies have shown that polymer-encapsulated cells can deliver ciliary neurotrophic factor (CNTF) continuously with therapeutic efficacy in rodent models (Emerich et al., 16 J. Neurosci. 5168–81 (1996)). Clinical trials support the safety of chronic CNTF delivery into the human central nervous system (CNS) with polymer-encapsulated cells (Aebischer et al, 7 Hum. Gene Ther. 851–60 (1996), Aebischer et al., 2 Nature Medicine 696–9 (1996)). However, a major challenge in translating such successes from rodent models to humans is ensuring long-term cell viability in encapsulated devices in vivo.